This invention relates to a photo-electric conversion and more particularly to a photo-electric element and an array formed thereby suitable for a photosensitive sensor, a photo-electric image pick up tube and a photo-electric cell which are used for the OCR apparatus (Optical Character Reader) or the like employed in the terminals of a computer or a facsimile.
Heretofore, an integrated silicon device having a number of photodiodes and the driving circuits thereof was mainly used as a photo-electric converting device of photosensitive sensor of the facsimile apparatus or the like. Where characters or the like in original are to be detected through the facsimile, the image of the original was projected in a reduced size onto an integrated photo-electric converting array by means of an optical means such as a lens. Due to the necessity of employing of such optical means, it was difficult to miniaturize the whole of the apparatus and, because of use of monocrystalline silicon, the apparatus was expensive.
In order to solve the problems as discussed above, exploitation of one-dimensional or two-dimensional photo sensor which senses the object to be sensed such as the original in a position very closed to it, by the dimensions ratio of about 1:1, and which is of substantially the same size as that of the object to be sensed, has long been desired but no such photo sensor with satisfactory characteristics has yet been exploited. Heretofore the photo sensor comprising of a material, such as amorphous chalcogenide material was developed. It utilized changes in the backward current with respect to the light irradiation under the condition where the rectifying backward biasing voltage is applied, as shown by the range "I" in FIG. 1. It was found, however, that in such a photo-electric converting apparatus which carries out sensing under the condition where the backward biasing voltage is applied, higher S/N ratio is not achieved due to the flow of the dark current; the apparatus necessitates the backward biasing circuit; and should the amorphous chalcogenide layer reached to the temperature of the order of 60.degree. C.-70.degree. C., it would be crystallized. Due to these drawbacks that are always accompanied by the apparatus as discussed above, it was not possible to obtain a high reliability for the apparatus. It is to be noted that all of the photo sensors heretofore available employing semiconductor materials other than the aforementioned amorphos chalcogenide, also utilize the backward voltage with respect to the light irradiation under the state of the rectifying backward biasing.
Of the semiconductor materials to be used for the photo sensor, a semiconductor having a hetero-junction of selenium (Se) and cadmium selenide (CdSe) is heretofore known, the semiconductor is comparatively reasonable in price, and excellent in its conversion efficiency, and such semiconductor has already been used for a photo-cell. As is shown in FIG. 2, such photo-cell is produced by the following steps: the surface of an iron or aluminium substrate 1 is roughened by a sandblast or the like to obtain good adhesion of a selenium layer, to the substrate, then a nickel plated layer 2 is provided on the substrate, then a selenium layer 3 is formed by a vacuum deposition on the layer 2, then a transparent electrode 4 is formed by a thin deposition of cadmium or the like by means of a vacuum deposition, and a collecting electrode 5 is formed around the transparent electrode 4. The photo-cell with such construction, however, involves the problems as follows.
The first problem is that, due to the roughened surface of the substrate, there is roughness of from 10 .mu.m to a few tens .mu.m, so it necessitated a selenium layer of from 50 to 60 .mu.m in order to obtain a flat surface of the selenium layer. This results in the increase in the internal resistance which is not preferable to the photo-electric converting element and the array thereof.
The second problem is that, due to the heavy selenium layer, fine fabrication is not available. The area of the selenium layer should critically be about 1 mm.sup.2 and it is extremely difficult to obtain the photo-electric converting element and the array thereof which will satisfy the requirement as intended by the present invention.
The third problem is that, due to the use of non-transparent substrate, the incident direction of the light is limited to one direction and, particularly the material for the transparent electrode should be cadmium only, therefore it is not possible to satisfy the requirements of decreasing in the electric resistance and increasing in the transparency, both are in conflict with each other.
An additional drawback in the photo-electric converting device heretofore exploited is that where a substrate with a flat surface particularly a sheet of glass or the like is used, a selenium layer deposited on the substrate will be caused to peel off the substrate when the layer is crystallized and, hence, it is not possible to fixedly deposit the selenium layer on the substrate. In order to overcome this problem, it has been carried out to form the selenium layer followed by the deposition of tellurium onto the glass substrate. Even in such a construction, the thickness of the selenium layer is limited and, further, the internal resistance is increased by the tellurium deposited.